1. Field of the Invention
The present invention relates to a zincate type zinc alloy electroplating bath prepared by dissolving a salt of Fe(II, III) or Ni(II) in the presence of a chelating agent and by adding brightening agents.
2. Description of the Prior Art
It is well known that combined deposition of iron (Fe) or nickel (Ni) in zinc plated coatings results in improved corrosion resistance thereof. Various types of zinc alloy electroplating baths have been proposed heretofore to permit combined deposition of Fe or Ni.
In various zinc alloy electroplating baths which have been reported, a typical electroplating bath which permits deposition of an alloy of Zn-Fe or Zn-Ni is a zincate type zinc alloy electroplating bath which contains a salt of Fe or Ni in the presence of a chelating agent. Such a zincate type Zn-Fe alloy electroplating bath is described in Technical Reports published by Kyoto Prefectural Small Business General Guidance Division; Volume 7 (1979), pages 20 to 26, Volume 8 (1980), pages 43 to 45 and Volume 9 (1981), pages 32 to 37, and in Practical Surface Technique, Volume 29 (1982), No. 8, pages 383 to 387, published by The Surface Finishing Society of Japan. Also, a zincate type Zn-Ni alloy electroplating bath is described in Digest of the 70th Scientific Lecture Meeting (1984), pages 28 to 29, published by The Surface Finishing Society of Japan.
In many cases, zinc electroplated coatings are subjected to chromate treatment for the purpose of improving corrosion resistance. However, higher deposition ratio of Fe or Ni prevents application of a good chromate coating. On the contrary, lower deposition ratio prevents satisfactory improvement of corrosion resistance. Therefore, the deposition ratio of Fe or Ni must be relatively closely controlled. It has been reported from results of researches that the desirable range of the deposition ratio is 0.1 to 5% for Fe and 2 to 20% for Ni.
In regard to appearance, the plated surface is desired to have excellent brightness, because the quality of the plated surface obtained by post-treatment such as chromate treatment largely depends upon the brightness of the plated surface before such post-treatment.
In electroplating processes, it is technically difficult to maintain a uniform current density over the whole surface of an article to be plated. For example, when articles having projected portions and recessed portions are plated, the current density on the projected portions becomes higher than that on the recessed portions, and it is difficult to obtain a substantially equal current density on these two portions. Therefore, the important condition required for a practical zinc alloy electroplating bath is to assure provision of a substantially constant alloy composition ratio in a wide range of current density as well as provision of coatings having excellent brightness in a wide range of current density.
Practical usability of the conventional Zn-Fe and Zn-Ni alloy electroplating baths as described above has been examined, with the result that, in these alloy electroplating baths, the range of the current density which allows provision of bright coatings is narrow, and the deposition ratio of Fe or Ni to Zn largely depends upon the level of the current density.
In order to avoid the disadvantages of the conventional alloy electroplating baths, the present inventors have developed a zinc alloy electroplating bath as follows. The bath is a zincate type zinc alloy electroplating bath containing a Zn compound, an alkali hydroxide, a salt of Fe(II, III) or Ni(II), a chelating agent for dissolving the salt of Fe(II, III) or Ni(II) and a brightening agent. The brightening agent used here is an alkylated polyalkylene polyamine in which basic nitrogen atoms of polyalkylene polyamine are partially alkylated by alkyl groups of C.sub.1 to C.sub.3. This zinc alloy electroplating bath is disclosed in Japanese Laid-Open Patent Publication No. 62-238387. It has been found that, from such a zinc alloy electroplating bath, bright coatings may be obtained in a relatively wide range of current density, and variations in deposition ratio of Fe or Ni in accordance with the level of the current density is small. In the zinc alloy electroplating bath, the range of the current density assuring provision of satisfactorily plated coatings is 0.2 to 15 A/dm.sup.2. The present invention has been worked out to further improve the electroplating bath as disclosed in the above Japanese Laid-Open Patent Publication No. 62-238387.